Geo log i cal Quar terly, 2016, 60 (4): 801–810 DOI: http://dx.doi.org/10.7306/gq.1314
X-ray mi cro com puted to mog ra phy char ac teri za tion of po ros ity in Rotliegend sand stones on the north ern slope of the Wolsztyn Ridge, west ern Po land
Anna POSZYTEK1, *, Zbigniew MIKO£AJEWSKI2 and Marek DOHNALIK3
1 Uni ver sity of War saw, Fac ulty of Ge ol ogy, In sti tute of Geo chem is try, Min er al ogy and Pe trol ogy, ¯wirki i Wigury 93, 02-089 Warszawa, Po land
2 Pol ish Oil and Gas Com pany (PGNiG S.A.), Ex plo ra tion and Pro duc tion Branch, Pi³a, pl. Staszica 9, 64-920 Pi³a, Po land
3 Oil and Gas In sti tute – Na tional Re search In sti tute, Well Log ging De part ment, Lubicz 25a, 31-503 Kraków, Po land
Poszytek, A., Miko³ajewski, Z., Dohnalik, M., 2016. X-ray mi cro com puted to mog ra phy char ac teri za tion of po ros ity in Rotliegend sand stones on the north ern slope of the Wolsztyn Ridge, west ern Po land. Geo log i cal Quar terly, 60 (4): 801–810, doi: 10.7306/gq.1314
Nat u ral gas in the Pol ish Rotliegend Ba sin oc curs in po rous and per me able ae olian sand stones, and traps are mostly struc - tural. Lithological traps are rare and oc cur on the north ern slope of the Wolsztyn Ridge where flu vial and ae olian sand stones over lap with al lu vial fa cies. Both flu vial and ae olian sand stones are res er voir rocks in this area. The X-ray mi cro com puted tomography (MCT) re sults and mi cro scopic ob ser va tions in di cate that the sand stones in lith o logic traps on the north ern slope of the Wolsztyn Ridge form highly com part men tal ized ver ti cal res er voir rocks com posed of four types of sand stones.
The pro files are dom i nated by very low-po ros ity flu vial sand stones (F2), and low-po ros ity ae olian and flu vial sand stones (A2, F1). The A1 type of sand stones with high po ros ity (10%) oc curs only in some sec tions of the pro files. The most im por tant diagenetic pro cesses that re duced po ros ity were com pac tion and ce men ta tion by car bon ate ce ments. All stud ied sand - stones were sub jected to the same diagenetic pro cesses. How ever, each of the pro cesses ran with vary ing in ten sity in dif fer - ent types of sand stones. De tailed anal y ses of pore dis tri bu tion by MCT meth ods with re spect to pri mary depositional or lithofacies ef fects, and sec ond ary diagenetic ef fects, help to un der stand the 3D ge om e try of pores and pore shape-size dis - tri bu tions. The re sults can be used in the stud ies of other sand stones with a dif fer ent or i gin and age.
Key words: Rotliegend, lith o logic traps, sand stones, po ros ity, Wolsztyn Ridge, com puted microtomography.
INTRODUCTION
The Up per Rotliegend suc ces sion has been a tar get for nat u - ral gas ex plo ra tion in west ern Po land for over fifty years (Fig. 1;
P. Karnkowski, 1999; Karnkowski, 1999; Wolnowski, 2004), and ae olian sand stones are the most im por tant res er voir rocks (Karnkowski, 2007; Kiersnowski et al., 2010a). In the Poznañ Ba - sin (Fig. 1), ae olian sand stones have po ros ity >20% and per me - abil ity >1000 mD over a wide area (Buniak et al., 2009; Papiernik et al., 2010; Such et al., 2000, 2010). Most nat u ral gas ac cu mu - la tions in the Poznañ Ba sin oc cur in struc tural traps (Karnkowski, 1999). Pre vi ous stud ies have in di cated that a de crease in the sand stones’ po ros ity and per me abil ity oc curs in the south of the ba sin and is linked with the oc cur rence of flu vial and al lu vial de - pos its near the Wolsztyn Ridge (Buniak et al., 2009). How ever, com bi na tion (lithological-struc tural) traps oc cur on the north ern mar gin of the Wolsztyn Ridge, for ex am ple at the Ujazd and
Paproæ gas fields (Kwolek et al., 2004), where res er voir rocks in - clude both ae olian and flu vial sand stones. No de tailed stud ies of the po ros ity dis tri bu tion in ae olian and flu vial sand stones on the north ern mar gin of the Wolsztyn Ridge have so far been pub - lished. The aim of this study was there fore to in ves ti gate the po - ros ity within these sand stones us ing mi cros copy ob ser va tion and mod ern anal y ses as com puted micro tomography. Qual i ta tive and semi-quan ti ta tive anal y ses of com pu ted microtomography with re spect to pri mary depo si tional or lithofacies ef fects, and sec ond ary diagenetic ef fects, help to un der stand the po ros ity dis tri bu tion in tested lithological traps.
GEOLOGICAL SETTING
The South ern Perm ian Ba sin in west ern and cen tral Eu rope ex tends from Great Brit ain to Po land and con tains im por tant re - serves of nat u ral gas de rived from Car bon if er ous shale and coal source rocks (Glen nie, 1990; Ziegler, 1990; Karnkowski, 1999; Doornenbal and Stevenson, 2010). The South Perm ian Ba sin sys tem is di vided into a num ber of smaller bas ins, in clud - ing the Pol ish Perm ian Ba sin (Fig. 1A) which de vel oped as a re - sult of post-Variscan dextral transtension (Nikishin et al., 2002;
Lamarche and Scheck-Wenderoth, 2005).
* Corresponding author, e-mail: anna.poszytek@uw.edu.pl
Received: April 28, 2016; accepted: August 22, 2016; first published online: September 26, 2016
In the Pol ish Perm ian Ba sin, Lower Rotliegend (Autunian) sed i men tary and vol ca nic rocks are over lain by Up per Rotliegend rocks de pos ited un der arid and semi-arid cli ma tic con di tions (e.g., Karnkowski, 1999; Kiersnowski et al., 2010b;
Kiersnowski, 2013). Con ti nen tal Rotliegend sed i men ta tion was ter mi nated by the Zechstein ma rine trans gres sion and by Kupferschiefer sed i men ta tion (e.g., Jerzykiewicz et al., 1976;
Peryt, 1976; Nemec and Porêbski, 1977; Karnkowski, 1986;
Kiersnowski, 1997; Wag ner and Peryt, 1997; Michalik, 2001; cf.
Peryt et al., 2012). The Kupferschiefer (T1) is over lain by the Zechstein Lime stone (Ca1) and then PZ1 evaporites (Dyja - czyñski and Peryt, 2014).
Dis tri bu tion of the Up per Rotliegend sed i men tary de pos its, their fa cies and thick ness were con trolled by the Wolsztyn Ridge. The Wolsztyn Ridge is a tec tonic horst which di vides the south ern sec tion of the Pol ish Rotliegend Ba sin into the Poznañ sub-ba sin to the north, and the south ern Silesian sub-ba sin (Fig. 1B). The ridge is com posed of folded, faulted and eroded Visean to Namurian flysch type rocks capped by a thick cover of Up per Car bon if er ous–Lower Perm ian vol ca nic rocks (Mazur et al., 2003, 2006; Dadlez, 2006; Geisler et al., 2008; Kiersnowski et al., 2010b). Wolsztyn Ridge was the source area for al lu vial and flu vial sed i ments dur ing the sed i men ta tion of the Up per Rotliegend (Karnkowski, 1987, 1994; Weihe, 1997; Kiersno - wski et al., 2010b). Close to the Wolsztyn Ridge, sev eral sed i - men tary cy cles of prograding al lu vial fans and flu vial de po si tion
can be dis tin guished (Kiersnowski et al., 2010b). These de pos - its are fre quently interbedded with ae olian sands (Fig. 2).
The Poznañ Ba sin, lo cated in the cen tral part of the Pol ish Rotliegend Ba sin, is char ac ter ized by large thick nesses of the Up per Rotliegend, ex ceed ing 1 km. The Up per Rotliegend sec - tion is com posed of al lu vial and flu vial rocks with a con sid er able pro por tion of ae olian sand stones in the cen tral part of the Poznañ Ba sin. Ae olian sand stones are the main res er voir rocks for gas ac cu mu la tion (Jarzyna et al., 2009; Papiernik et al., 2010; Poszytek, 2014). The res er voir prop er ties of ae olian sand stones are char ac ter ized by very good po ros ity (>20%) and per me abil ity (>1000 mD) and form a wide zone in the cen - tral part of the ba sin. Pre vi ous stud ies (Buniak et al., 2009) in di - cated that the de crease in the po ros ity and per me abil ity val ues of ae olian sand stones to the south and north in the Poznañ Ba - sin is linked with the oc cur rence of flu vial de pos its near the Wolsztyn Ridge or with the greater sub si dence, which is in turn as so ci ated with more in tense com pac tion of ae olian sand - stones.
Nu mer ous con ven tional gas res er voirs have been rec og - nized in Po land (Fig. 1) within the po rous and per me able ae - olian sand stones of the Noteæ For ma tion sealed by Zechstein evaporites and im per me able claystones (e.g., Kwolek et al., 2004; Wolnowski, 2004; Karnkowski, 2007; Kiersnowski et al., 2010a, b). Huge amount of geo log i cal in for ma tion from many oil bore holes re sulted in num ber of pub li ca tions ded i cated on Fig. 1. Lo ca tion map of the gas fields within the top part of the Up per Rotliegend de pos its in the Pol ish Perm ian Ba sin
A – af ter Kiersnowski in Doornenbal and Stevenson (2010); B – af ter Kwolek et al. (2004)
Rotliegend sed i men ta tion (Karnkowski, 1987, 1994; Kiersno - wski, 1997; Buniak et al., 1999; Pokorski and Kiersnowski, 1999; Szwarc and Kiersnowski, 1999; Kiersno wski and Buniak, 2006; Kiersnowski et al., 2010b), its pe trog ra phy (Rochewicz, 1980; Muszyñski and Rydzewska, 1986; Maliszewska and Kuberska, 1996; Gregosiewicz and Protas, 1997), res er voir prop er ties and their re la tion ship to lithofacies (Such, 1996;
Such et al., 2000; Buniak et al., 2009), and ex plo ra tion of tight gas (Poprawa and Kiersnowski, 2008, 2010; Such et al., 2010).
Lithological and struc tural traps oc cur on the north ern mar - gin of the Wolsztyn Ridge (Fig. 1B) at depths be tween 2,400 and 2,800 m (Fig. 3). Lithological traps are de ter mined by a com bi na tion of the ex tent line of po rous, per me able sand stones (ae olian and flu vial) and the seal sur face of im per me able clastic de pos its of the Rotliegend (Fig. 3; Karnkowski, 1985; Kwolek et al., 2004). Lithological traps with gas de pos its on the north ern mar gin of the Wolsztyn Ridge were dis cov ered in the 1970s (Ujazd gas field) and 1980s (Paproæ gas field). Nat u ral gas re - serves are es ti mated at 3 bil lion m3 in the Ujazd gas fields and 4.5 bil lion m3 in the Paproæ gas fields (Karnkowski, 1999). Cur - rently, ex plo ra tion has been fo cused on the north ern mar gin of the Wolsztyn Ridge to rec og nize the ex ist ing gas de pos its and dis cover new gas fields (Kwolek et al., 2004). How ever, there are no de tailed pub li ca tion on the res er voir prop er ties and in ter - pre ta tion of the fac tors that played a ma jor role in the dis tri bu - tion of res er voir prop er ties.
MATERIALS AND METHODS
Data for this study came from six bore holes drilled through the last de cade (Czarna Wieœ 4, Czarna Wieœ 5, Czarna Wieœ 6, Parzêczewo 1, Parzêczewo 2, Cicha Góra 9) lo cated be - tween the Paproæ and Ujazd gas fields. Lithofacies and the depositional en vi ron ment of the rocks were in ter preted dur ing mac ro scopic ob ser va tion of cores. Later, 37 sam ples of ae olian and flu vial sand stones from 2,550–2,700 m depth were col - lected, based on struc tural and tex tural dif fer en ti a tion of rocks;
in clud ing sand stones, fine- to coarse-grained, fine to poorly sorted, mas sive, cross bed ded and hor i zon tal bed ded. Next 37 core plugs (2 ´ 0.5 cm) were pre pared for mi cro com puted to - mog ra phy (MCT). Thin sec tions were cut from the plugs af ter com puted microtomography anal y ses and were used for mi cro - scope ob ser va tion.
Mi cro scope anal y ses in ves ti gated the sand stones’ com po - si tion, grain sort ing, grain con tacts and the pres ence of authi - genic ce ments, as well as the oc cur rence of pri mary (depo - sitional) and sec ond ary (diagenesis-re lated) po ros ity. A scan - ning elec tron mi cro scope (JSM6380 LA) was used to in ves ti - gate the de tailed char ac ter is tics of the pore sys tem us ing the ImageJ programme, and bi nary im ages were used for qual i ta - tive and quan ti ta tive pore de scrip tion.
The pore dis tri bu tion of the Rotliegend sand stones was also in ves ti gated us ing MCT. X-ray MCT (Van Geet et al., 2001) is a non-de struc tive tech nique al low ing 3D vi su al iza tion of the in ter - nal struc ture of an ob ject (Ketcham and Carlson, 2001; Stock, 2008; Zapalski and Dohnalik, 2013; Couves et al., 2016). Fur - ther anal y sis of a 3D im age of a rock sam ple can pro vide char - ac te r iza tion of the qual i ta tive and quan ti ta tive pa ram e ters of the pore space (Cnudde et al., 2009; Fusi and Mar ti nez-Mar ti nez, 2013; Agbo gun et al., 2013). The MCT re sults were sup ported by de tailed sam ple de scrip tions us ing an op ti cal mi cro scope and a SEM.
Dur ing X-ray MCT anal y ses, X-ray are at ten u ated dur ing sam ple pen e tra tion and reach the de tec tor sur face where their in ten sity is con verted into an elec tri cal sig nal which cre ates a dig i tal im age known as a pro jec tion. The sam ple ro tates by 360°, and sev eral hun dred pro jec tions are re corded at dif fer ent an gu lar po si tions.
A fil tered back-pro jec tion al go rithm is then used to re con - struct the 3D struc ture of the ob ject from the pro jec tions, and a dig i tal im age is pro duced to il lus trate the vari abil ity of the lin ear X-ray mi cro com puted to mog ra phy char ac teri za tion of po ros ity in Rotliegend sand stones... 803
Fig. 2. Up per Rotliegend lithofacies in the Cicha Góra-9 bore hole lo cated close to the Wolsztyn Ridge, cy clic de po - si tion in ter preted based on Kiersnowski et al. (2010b)
ab sorp tion co ef fi cient (Stock, 2008). The im age con sists of voxels which are the small est el e ments of a spa tial im age and ap prox i mately cor re spond to pix els in 2D graphics. To cal cu late the po ros ity of the rock im age, a binarisation pro cess is re - quired; all the voxels rep re sent ing voids are as signed to one im - age layer, whereas all other voxels are as signed to a sec ond layer rep re sent ing the rock ma trix. The po ros ity is cal cu lated as the ra tio be tween the vol ume of pore-struc ture voxels to the to - tal vol ume of the sam ple.
MCT mea sure ments were made by the Oil and Gas In sti - tute in Cra cow us ing a Bench top CT160Xi (Nikon) X-ray microtomograph. The X-ray source in stalled in the ap pa ra tus emits a con i cal beam of X-ray with an ac cel er a tion volt age in the range of 40–160 kV and up to 3 mm res o lu tion (the res o lu - tion of the ex am ined sam ples was 6 mm). The ac cu racy of the mea sure ments de pends on the num ber of pro jec tions, the num ber of av er ag ing pro jec tions at each an gu lar po si tion, as well as the ex po si tion/ex po sure time. These three pa ram e ters may sig nif i cantly in crease the sig nal-to-noise ra tio in the pro jec - tions reg is tered.
The re sults of microtomographic stud ies are pre sented in the form of 3D visu ali sa tions of the pore space. 3D visu ali sa tion en ables a qual i ta tive anal y sis of the char ac ter is tics of the pore space, in clud ing as sess ment of the ani so tropy of the pore net - work pat tern in the sam ple (Svitelman and Dinariev, 2013).
Charts show ing the pore dis tri bu tion dis play the quan ti ta tive char ac ter is tics of each sam ple and a com par i son with other sam ples. Quan ti ta tive eval u a tion of po ros ity used MAVI soft - ware (Mod u lar Al go rithms for Vol ume Im ages, Fraun hofer In sti - tute for In dus trial Math e mat ics ITWM).
3D visu ali sa tion of the pore space used AVIZO 6.3 soft ware (reg is tered trade mark, de vel oped by Vi su al iza tion Sci ences Group, now FEI), and the pore net work was sub di vided into sub groups, each sub group rep re sent ing a set of in ter con nected pores which do not com mu ni cate with other sub groups. The sub groups have been di vided into classes ac cord ing to their vol ume. The unit used to de scribe vol ume classes is the voxel, which in this study has the di men sions of 6 × 6 × 6 mm so 1 voxel
= 216 mm3 and this is the small est vol ume which can be di ag - nosed in this study. Six vol ume classes were dis tin guished in the po ros ity sub groups ac cord ing to a log a rith mic scale, and the sub groups were marked with spe cific colours:
I. 1–9 voxels – vi o let, II. 10–99 voxels – blue, III. 100–999 voxels – red, IV. 1000–9999 voxels – green, V. 10 000–99 999 voxels – white, VI. >100 000 voxels – yel low.
Charts of pore dis tri bu tion were con structed based on the di vi sion of the pore space into sub groups. The charts show the
Depth [m]
Distance [m]
Fig. 3. Ve loc ity model of Me so zoic and Perm ian de pos its in the Wycis³owo-Ksi¹¿ re gion (NE slope of the Wolsztyn High; Kwolek et al., 2004)
per cent age dis tri bu tion of par tic u lar vol ume classes of the pore net work, in di cat ing which class dom i nates in the sam ples. A dom i nance by the high pore classes (in ter con nected pores) re - sults in the rock hav ing better res er voir pa ram e ters (Dohnalik et al., 2010; Zalewska and Dohnalik, 2011). Quan ti ta tive anal y ses of the vari abil ity of the pore net work were based on charts show ing the dis tri bu tion of pore classes.
RESULTS
DEPOSITIONAL SETTING
The Rotliegend de pos its on the north ern mar gin of the Wolsztyn Ridge are char ac ter ized by a wide range of clastic fa - cies. The study pro files con sist of thick (up to 100 m in their lower part) al lu vial fan sed i ments com posed of con glom er ates, over lain by finer-grained al lu vial and flu vial de pos its (sand - stones and mudstones). These rocks are interbedded with ae - olian sand stones. Flu vial sand stones con sist of me dium- and coarse-grained sand stones which are poorly sorted and cross - -bed ded, or lo cally mas sive, and are in ter preted as chan nel and overbank fa cies. Ae olian sand stones con sist of me dium- and fine-grained sand stones with in clined (up to 15°) bed ding sur - faces, in ter preted as dune fa cies. The thick ness of the flu vial and ae olian sand stones var ies from a few to tens of metres (Fig. 3).
Pre vi ous stud ies (Kiersnowski et al., 2010b) in di cated that the al lu vial and flu vial de pos its with interbeds of ae olian de pos - its are char ac ter ized by rapid changes in sed i men tary en vi ron - ment con di tions. Al lu vial fans rep re sent a syn- or post-tec tonic sed i men tary re sponse to fault ac tiv ity. Four sed i men tary cy cles were dis tin guished in the study area based on cy clic de po si tion anal y ses pro vided by Kiersnowski et al. (2010b) (Fig. 2). Dur ing tec tonic qui es cence and low ac tiv ity on the prograding al lu vial fan, less coarse flu vial de pos its dom i nated. These de pos its are fre quently interbedded with ae olian sands as a re sult of al ter - nat ing arid and hu mid cli ma tic pe ri ods and due to eas ier mi gra - tion of ae olian dunes up the al lu vial fan slopes (Kiersnowski et al., 2010b).
MICROSCOPY OBSERVATION
Tested ae olian sand stones are rep re sented by quartz and sublithic arenites (ac cord ing to the clas si fi ca tion of Pettijohn et al., 1972; Fig. 4). The grains are loosely to densely packed, and the inter gra nu lar con tacts are point or pla nar, in di cat ing phys i - cal com pac tion. Grain sort ing is good to mod er ate. Some sam - ples are char ac ter ized by a high po ros ity value (>10% – A1 sand stones). The high po ros ity was ac com pa nied by low con - tents of ce ment which was mainly in the form of clay-he ma tite grain coat ings and rare quartz coat ings on quartz grains. Much of the ae olian sand stones were char ac ter ized by in tense ce - men ta tion by car bon ate (cal cite) and clay (illite and kaolinite) ce ments, which con sid er ably re duced pri mary po ros ity (Fig. 5 – A2 sand stones).
Ac cord ing to the clas si fi ca tion of Pettijohn et al. (1972), the flu vial sand stones in clude lithic, sublithic and subarkosic arenites (Fig. 4). The flu vial granulometry is char ac ter ized by poor sort ing (Fig. 5). The rock clasts in clude mainly vol ca nic rocks from the ero sion of the Wolsztyn Ridge. The grains are densely packed, and the inter gra nu lar con tacts are point or pla - nar, in di cat ing phys i cal com pac tion. The frame work of ten con - tains ma trix and car bon ate ce ment (Fig. 5), which fill the inter -
gra nu lar po ros ity partly or com pletely. Flu vial sand stones are char ac ter ized by low and very low po ros ity (<5% – F1 and F2 sand stones). Po ros ity is both inter gra nu lar and intragranular.
Intragranular po ros ity plays a smaller role than inter gra nu lar po - ros ity.
Com pared to ae olian sand stones, flu vial sand stones are char ac ter ized by poorer grain sort ing and lower po ros ity (Figs. 5 and 6).
MICRO COMPUTED TOMOGRAPHY
Based on qual i ta tive and semi-quan ti ta tive anal y ses of MCT re sults all sam ples were di vided into four groups, de pend - ing on their sed i men tary or i gin and char ac ter of po ros ity. The ae olian sand stones were sub di vided into high- (A1) and low-po - X-ray mi cro com puted to mog ra phy char ac teri za tion of po ros ity in Rotliegend sand stones... 805
Fig. 4. Min eral com po si tion of the skel e tal grains of ae olian and flu vial sand stones in the QFL clas si fi ca tion di a gram
of Pettijohn et al. (1972)
Fig. 5. Vari abil ity of li thol ogy in ae olian and flu vial sand stones with re spect to the dis tin guished
types of sand stone
ros ity (A2) fa cies. Flu vial sand stones can be sub di vided into two: low (F1) and very low po ros ity (F2) (Figs. 5 and 6).
A1 sand stones. A1 sand stones are char ac ter ized by well- de vel oped pri mary inter gra nu lar po ros ity (up to 10%), good grain sort ing and low ce ment con tent (Fig. 5). Plots of pore vol - ume class dis tri bu tion are char ac ter ized by a low pro por tion of classes I, II and III rel a tive to the higher classes (Fig. 6). Class VI pore vol ume oc curs in all sam ples. It is of ten dom i nant or co-oc curs with an other dom i nant, class V pore vol ume. Class VI pore vol ume is re lated to the well-pre served pri mary inter gra - nu lar po ros ity. Dis tri bu tion of po ros ity is partly lim ited by com - pac tion and ce men ta tion pro cesses (Fig. 5). Ae olian sands were most likely char ac ter ized by a well-de vel oped pore net - work af ter sed i men ta tion. Good grain sort ing had a de ci sive role in shap ing the orig i nal high po ros ity. Com pac tion and ce - men ta tion by early car bon ate ce ments caused frag men ta tion of the pri mary pore net work. As a re sult, diagenetic pro cesses caused the trans fer of dominants from class VI pore vol ume to a lower class (V and IV). The pres ence of the low est pore vol ume classes (I and II) is con nected with the pres ence of clay min er - als (illite and kaolinite) in pores, which are authigenic min er als and formed very fine, dis persed microporosity (Figs. 5 and 6).
Such microporosity may also be formed by grain dis so lu tion, but no dis solved grains were ob served in this group of sand - stones. Hence, com pac tion and ce men ta tion by car bon ate and clay min er als formed the cur rent val ues of po ros ity. In ad di tion to the pri mary char ac ter is tics of the sed i ment, it is ob served that the dis tri bu tion of po ros ity is char ac ter ized by clear ani so tropy
in the 3D visu ali sa tion (Fig. 6). It is the re sult of the pres ence of cross bed ding and the oc cur rence of interlayered fine- and me - dium-grained sand stones. Me dium-grained sand stones are char ac ter ized by a higher class VI pore vol ume than fine - -grained sand stones.
A2 sand stones. A2 sand stones are char ac ter ized by a dom i nance of inter gra nu lar po ros ity (up to 5%), sim i lar to A1 sand stones. How ever, the pores are much smaller and the pore dis tri bu tion is re duced by in tense com pac tion of poorly sorted grains and the oc cur rence of high vol ume of ce ments (Fig. 5).
The plots of pore vol ume class dis tri bu tion are char ac ter ized by the lack of class VI and the dom i nance of class III or IV (Fig. 6).
It is likely that class VI never formed as a re sult of poor sort ing. If class VI was cre ated, it was quickly re duced to the lower classes as a re sult of com pac tion. In ad di tion, pri mary po ros ity was re duced by clay and car bon ate ce men ta tion. A higher por - tion of class I in A2 sand stones in di cates a higher con tent of clay min er als in the pore space. De pend ing on the sort ing of grains and the in ten sity of com pac tion and ce men ta tion pro - cesses, the dom i nant moved from class VI to classes III–IV, rel - a tive to A1 sand stone (Fig. 6). The 3D visu ali sa tion shows clear ani so tropy in the dis tri bu tion of po ros ity (Fig. 6) as so ci ated with the oc cur rence of cross bed ding, as ob served in A1 sand - stones.
F1 sand stones. F1 sand stones are char ac ter ized by the oc cur rence of pri mary, inter gra nu lar and sec ond ary po ros ity.
Sec ond ary po ros ity is as so ci ated with the dis so lu tion of feld - spar grains and vol ca nic and clastic lithoclasts (Fig. 5). The size Fig. 6. Vari abil ity of the po ros ity dis tri bu tion in the dis tin guished types of sand stones
of inter gra nu lar pores reaches over 100 mm and they are par - tially con nected with each other. Sec ond ary pores are smaller (<5 mm) and they are ran domly dis trib uted in the sam ple. To tal po ros ity reaches 3%. The plots of pore vol ume class dis tri bu - tion are char ac ter ized by a lack of class VI and dom i nance of class III or IV. The plots for F1 sand stones are very sim i lar to those for A2 sand stones (Fig. 6). F1 sand stones are poorly sorted, with a small share of ma trix in the inter gra nu lar pore space. The lack of class VI pore vol ume may be caused by poorer grain sort ing of F1 sand stones and a much less de vel - oped pore net work at the stage of sed i men ta tion. Poorer grain sort ing also fa voured the in ten sity of com pac tion pro cesses, which re sulted in fur ther deg ra da tion of the pore net work. In ad - di tion, car bon ate ce men ta tion min er als sig nif i cantly re duced the pri mary po ros ity (Fig. 5). Over 10% of the pores are re lated to class I pore vol ume, which is linked to the sec ond ary po ros ity as a re sult of the dis so lu tion of un sta ble grains (vol ca nic and clastic lithoclasts), and has cre ated small, iso lated pores. The un even dis tri bu tion of po ros ity is clear in the 3D visu ali sa tion, which is as so ci ated with in tense com pac tion and the pres ence of car bon ate ce ments (Fig. 6).
F2 sand stones. F2 sand stones are char ac ter ized by the dom i nance of very fine pores of <5 mm (Fig. 5). The sand stones are poorly sorted, with a high pro por tion of ma trix in the pore space. Grains are closely packed and the inter gra nu lar space is filled by car bon ate ce ments. Po ros ity does not ex ceed 1%. The plots of pore vol ume class dis tri bu tion are char ac ter ized by the lack of class VI pore vol ume and dis tinc tive pres ence of class I pore vol ume (Fig. 6). The dom i nant class I pore vol ume in di cates microporosity, which was con firmed by mi cro scopic ob ser va - tions. We have ob served sec ond ary po ros ity in the thin sec tions (Fig. 5) re lated to feld spar grain and lithoclast dis so lu tion and the pres ence of clay min er als–re sid uum af ter grain dis so lu tion. The 3D visu ali sa tion shows clear reg u lar ani so tropy in the dis tri bu tion of po ros ity as so ci ated with the oc cur rence of zones of strong com pac tion and car bon ate ce ments in the pore space.
DISCUSSION
Up per Rotliegend sand stones on the north ern mar gin of the Wolsztyn Ridge are char ac ter ized by much lower po ros i ties com pared to ae olian sand stones in the cen tral part of the Poznañ Ba sin and are linked with the oc cur rence of flu vial and al lu vial de pos its near the Wolsztyn Ridge (Buniak et al., 2009).
Ae olian sand stones are po ten tially the best res er voir rock among the clastic rocks due to their ini tial tex tural fea tures, the most im por tant of which is the good grain sort ing. How ever, only the A1 sand stones have high po ros ity (up to 10%) in the study area, due to good grain sort ing, mi nor com pac tion and low con tent of inter gra nu lar ce ment. Sort ing of the A2 sand - stones is much poorer, the grains are closely packed, and there is a high vol ume of car bon ate and clay ce ments in the pore spaces; as a re sult, po ros ity of the A2 sand stones is less than half that of the A1 sand stones. The flu vial F1 sand stones are sim i lar to the A2 sand stones, de spite their dif fer ent or i gins, and may in fact rep re sent flu vial sands which were re de pos ited by ae olian pro cesses dur ing dry pe ri ods. Sim i larly, some ae olian sands may have been re de pos ited by flu vial trans port dur ing wet pe ri ods (Kiersnowski et al., 2010b; Kiersnowski, 2013;
Poszytek, 2014), and some flu vial and ae olian sand stones may there fore have sim i lar lithologies and po ros ity dis tri bu tions. By con trast, F2 sand stones are char ac ter ized by a higher ma trix con tent and poorer grain sort ing and, as a re sult, the po ros ity of these sand stones is very low.
Car bon ate ce men ta tion is an im por tant pro cess for po ros ity re duc tion in the stud ied sam ples. Or i gins of car bon ate ce ments in sand stones can be dif fer ent de pend ing on the sed i men tary en vi ron ments and diagenetic his tory. Car bon ate ce men ta tion in flu vial sand stones is of ten early and strongly re duces po ros ity (Maliszewska and Kuberska, 1996; Kuberska, 2004; Rusek et al., 2005), but car bon ate ce men ta tion in ae olian sand stones can be both early and late-stage.
Early car bon ate ce ments may have been formed as a re sult of the in fil tra tion of ma rine wa ters from over ly ing Zechstein car - bon ates (e.g., Buniak et al., 1999, 2009; Kiersnowski et al., 2010b; Poszytek, 2014). This may have oc curred in the up per - most Rotliegend sand stones, where per me able ae olian sand - stones are over lain by Zechstein de pos its; the zone of ce men - ta tion may be up to tens of metres thick (Poszytek, 2014).
Car bon ate ce ments also oc cur in ae olian sand stones be low the in fil tra tion zone and are prob a bly late-stage ce ments as so ci - ated with the flu ids mi gra tion trough gaps and fis sures de vel oped in fault zones (Kiersnowski et al., 2010a). The stud ied sam ples have not been ex am ined for the age of car bon ate ce ments (for ex am ple by sta ble iso tope stud ies). Low con tent of car bon ate ce - ments in ae olian sand stones and the oc cur rence of low-per me - abil ity sand stones in the top most part of the pro files in this area in di cate that this ce ment did not orig i nate from the in fil tra tion of Zechstein wa ter. It could be an early ce ment, but the source of wa ter was rather a flu vial en vi ron ment. Both flu vial and ae olian sand stones could be ce mented by late car bon ate ce ments in re - la tion to many dis lo ca tions in the study area.
CONCLUSIONS
Our re sults in di cate that the res er voir rock in the stud ied lithological traps is clearly sub di vided into zones of good po ros - ity (A1 sand stones), low po ros ity (A2 and F1 sand stones), and very low po ros ity (F2 sand stones) (Fig. 7). Dif fer ent types of sand stones al ter nate in the pro files, which are re flected in the seis mic im ages as ho mog e nized zones of rel a tively low po ros - ity. This causes great dif fi culty in iden ti fy ing the range of po rous and per me able sand stones within the traps on the north ern slope of the Wolsztyn Ridge (Kwolek et al., 2004).
All types of sand stones were sub jected to the same pro - cesses, but the pro cesses were of dif fer ent intensity. It should be noted that the in ten sity of diagenetic pro cesses was de ter - mined by the sed i men tary en vi ron ment (es pe cially sort ing). A1 sand stones are the best sorted, there fore the com pac tion ef fect is the small est com pared to other types of sand stones. Early ce men ta tion by car bon ate ce ment is also mi nor; there fore A1 type po ros ity is the most fa vour able. The other types of sand - stones are char ac ter ized by the worst sort ing, greater ef fect of com pac tion, and higher con tent of car bon ate cements, there - fore their porosity is lower.
The MCT re sults can be used to dis tin guish di verse pore shape classes which can be in ter preted as the re sults of pri - mary (sed i men tary) and sec ond ary (diagenetic) pro cesses (Fig.
8). The pres ence of class VI of pore vol ume (A1 sand stones) in - di cates the ex is tence of well-pre served pri mary po ros ity, which is due to good sort ing. The dom i nance of class IV of pore vol - ume (A2 and F1 sand stones) in di cates poorer sort ing of grains and more in ten si fied com pac tion (Fig. 8). A high con tri bu tion of class I and II pore vol ume (F1 and F2 sand stones) is as so ci ated with the pres ence of very fine po ros ity, which is the re sult of dis - so lu tion of the grains and the pres ence of clay min er als in the pore space. Poor sorted F2 sand stones con tain a huge amount of vol ca nic and clastic lithoclasts from ero sion of the Wolsztyn X-ray mi cro com puted to mog ra phy char ac teri za tion of po ros ity in Rotliegend sand stones... 807
Ridge. Dis so lu tion of these lithoclasts re sulted in an in crease of the amount of clay min er als in the ma trix of F2 sand - stones.
Pore sys tems cre ated from the MCT re sults can be used also for nu mer i cal mod el ling. It is im por tant that mod el ling re sults should be com pared with other meth ods, es pe cially mer cury porosi me - try. Hg-Cap-Curve can sup ple ment the data with a range of val ues smaller than the MCT resolution.
Sed i men tary con di tions (de ter min ing the grain sort ing and min eral com po si tion of the rock fab ric) and diagenetic pro - cesses (com pac tion, ce men ta tion and grain dis so lu tion) have cru cial in flu ence on the sand stone po ros ity. These fac tors play a de ci sive role in the cre ation of sand stone po ros ity re gard less of the or i - gin and age of the sand stone. Hence, the model pre sented in Fig ure 8 can be ap - plied to sand stones of dif fer ent or i gins and ages. It de scribes sand stone po ros - ity de pend ing on their lithological and tex - tural fea tures, and diagenetic pro cesses.
Fig. 8. Model of pore vol ume class dis tri bu tion in ae olian and flu vial sand stones Fig. 7. Up per Rotliegend lithofacies in the Cicha Góra 9
bore hole with sand stone types within the top part of the Up per Rotliegend
Ac knowl edge ments. The au thors would like to thank Prof.
R. Gaupp, Prof. T. Peryt, Dr. M. Kuberska, H. Kiersnowski and Prof. R. Sa³aciñski for the op por tu nity to dis cuss the re sults of our study. Polskie Górnictwo Naftowe i Gazownictwo S.A. has
kindly pro vided geo log i cal in for ma tion. The au thors would like to thank Dr. A. ¯yliñska for lan guage cor rec tions.
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